1. Field of the Invention
The present invention relates to a system for detecting a difference in road level, and more particularly to a system for determining if there is a difference in level on a road for vehicles by means of a linear longitudinal acceleration sensor.
2. Description of the Related Arts
Various anti-skid control systems have been proposed heretofore to prevent a wheel from being locked during a braking operation of a vehicle. For example, Japanese Patent Laid-open Publication No.5-294226 discloses an anti-skid control system which is adapted to detect a road giving a rapid variation of surface reaction force, such as a road with a difference in level, and prevent a hydraulic barking pressure from being reduced, or increase a pressure increasing gradient of the hydraulic braking pressure, thereby to improve the feeling in driving a vehicle. In the anti-skid control system as disclosed in the above-described publication, there is provided a road surface detection system which compares a wheel acceleration and a wheel speed which is reduced just before the ant-skid control starts, or just after it started, with respective predetermined levels, to determine if the road, on which the vehicle travels under the anti-skid control, is giving the rapid variation of surface reaction force.
According to the road surface detection system as disclosed in the above-described Japanese Patent Laid-open Publication No.5-294226, the difference in level on the road is detected on the basis of the reduced wheel acceleration and wheel speed, or recovered wheel acceleration and wheel speed. Therefore, vibration or the like of driven wheels might result in misjudgment of the difference in level on the road, especially on a road of low coefficient of friction. In the case where the above-described road surface detection system is used for the anti-skid control, it is necessary to prevent the wheels from being locked rapidly on the road of low coefficient of friction.